1. Field of the Invention
This invention relates to a low speed electron excited fluorescent material and a fluorescent display tube using such fluorescent material, and more particularly to a low speed electron excited fluorescent material which is improved in luminance, decreased in uneveness of luminance amd has excellent temperature characteristics, and a fluorescent display tube using such fluorescent material.
2. Description of the Prior Arts
Conventionally, a ZnO:Zn system fluorescent material has been widely known as a fluorescent material excited by a low speed electron ray accelerated with a voltage lower than 100 V to emit light. The ZnO:Zn system fluorescent material has been widely used as a fluorescent material for, for example, a fluorescent display tube, because it has several advantages in use such as, for example, of having a luminous threshold voltage of as low as 1-2 V and emitting light of a sufficient luminance with a voltage of as low as ten-odd V.
In addition, a fluorescent display tube manufactured using such low speed electron excited fluorescent material can be driven directly by a LSI due to its low voltage drivability and less power consumption. Also, the fluorescent display tube is of a self-light emitting type and provides color displays of green which can be easily observed. Thus, the fluorescent display tube has been extensively used for display systems in acoustic units and vehicles as well as in various types of electrical and electronic devices.
In the excitation of a fluorescent material with low speed electrons to allow the fluorescent material to emit light, the electrons impinging on the fluorescent material rarely penetrate into particles of the fluorescent material. And, it is observed that the electrons penetrate into the fluorescent material in depth as little as several to several tens .ANG. at most and luminescence of the fluorescent material takes place substantially on the surface thereof.
Thus, it is believed that the surface characteristics of the fluorescent material largely affects the luminous performance.
However, a fluorescent display device using such fluorescent material, for example, a fluorescent display tube is manufactured through various steps such as calcining, encapsulating, evacuating and the like. This results in the fluorescent material being exposed to various severe environments to cause the surface of the fluorescent material to easily undergo contamination, and various changes such as change in quality and the like.
This causes the fluorescent display tube manufactured to provide ununiform and/or insufficient luminous display. In particular, under the present circumstances that the fluorescent display tube is extensively applied to various fields such as a vehicle and the like as mentioned above, it is obliged to be often used in such a manner that it is exposed directly to the sunlight. This requires the fluorescent display tube to provide a display of luminance as high as possible, therefore, it is highly desired to avoid the lowering of luminance of the fluorescent material due to the contamination of the surface thereof.
In addition, the fluorescent material generally has a property that it is decreased in luminance when it is subjected to a high temperature for a long time. The luminance of the fluorescent material is also decreased at the operation under a high temperature due to the phenomenum that the luminance is attenuated by a high temperature. Thus, it is desired that the fluorescent material has luminous characteristics which are not adversely affected by such factors as mentioned above and good temperature characteristics.